The drain pipe under normal circumstances draws air when the engine is running, and this excess air is allowed for in the carburettor settings. If you blank off that hole/pipe, the engine will run too rich.

Good point. After plugging the drain hole. Adjust the carb mixture screw.
The engine will run smoother. It would be equivalent to having an air leak at the inlet manifold gasket.
A lot of improvements have been made in carburation since the 50's. Drain tubes were used mostly on engines that had S.U. carbs because gas would sometimes puddle in the inlet. I'm not sure why they were only used on the 1500 GT inlets and not added to other EnFo engines. Drain tubes were eliminated by the time the 1600 GT engines came out.